an instrument used in combat for the purpose of killing, injuring, or defeating an enemy. A weapon may be a shock weapon, held in the hands, such as the club, mace, or sword. It may also be a missile weapon, operated by muscle power (as with the javelin, sling, and bow and arrow), mechanical power (as with the crossbow and catapult), or chemical (chemical weapon) power (as with the rocket and missile and such guns as the cannon, rifle, and pistol). Weapons may also be classified as conventional, destroying by kinetic energy (as with the bullet) or by chemical energy (as with the bomb and grenade). The nonconventional category comprises nuclear weapons, such as the atomic bomb and thermonuclear bomb, as well as weapons of chemical warfare and biological warfare. All the aforementioned are offensive weapons, but such defensive measures as fortification, armour, and the helmet have also been considered weapons.

Weapons have been carried and delivered by a wide variety of vehicles, often called weapon platforms. These have included such naval craft as the ship of the line, battleship, submarine, and aircraft carrier; aircraft such as the fighter, bomber, and helicopter; and ground vehicles such as the chariot and tank.

The history of weapons of war is treated in the article military technology.